Container gardening is a great way to display your green thumb, and is especially convenient for apartment-dwellers and those with limited space. The down side is that potted flowers are much more vulnerable to cold than those planted in the ground. If you live in a climate where the temperature drops below freezing, the only way surefire way to protect potted flowers from freezing is to bring them indoors. If that isn't possible, there are several techniques that may give your potted flowers a chance of surviving the winter.
Plant the pots in soil, if possible. The ground will help to protect them from freezing, and you can dig the pots up when spring arrives.
Move potted flowers to a protected location, if you are unable to plant them in the ground. Put them next to a wall, in a corner, or under a tall tree. Group the pots together, with more cold-sensitive plants in the center of the group.
Wrap each pot with bubble wrap. If you don't have bubble wrap, surround them with burlap, and cover the burlap with plastic to keep it dry.
Tip the pots slightly on their side, if they aren't too tall. This will keep water from gathering in the pot.
Cover the pots with a piece of plastic large enough to go over the plants, and extend to the ground. If possible, put a light bulb under the plastic during hard freezes, but be sure the plastic doesn't touch the bulb. A string of Christmas lights will also help, and won't get as hot as a light bulb.